November 4, 2004
12:01 AM (EST)

News Release Number: STScI-2004-30

Hubble Spots Rare Triple Eclipse on Jupiter

November 4, 2004: At first glance, Jupiter looks like it has a mild case of
the measles. Five spots – one colored white, one blue,
and three black – are scattered across the upper half
of the planet. Closer inspection by NASA's Hubble Space
Telescope reveals that these spots are actually a rare
alignment of three of Jupiter's largest moons – Io, Ganymede,
and Callisto – across the planet's face. In this image, the
telltale signatures of this alignment are the shadows [the
three black circles] cast by the moons. Io's shadow is located
just above center and to the left; Ganymede's on the planet's
left edge; and Callisto's near the right edge. Only two of the
moons, however, are visible in this image. Io is the white circle
in the center of the image, and Ganymede is the blue circle
at upper right. Callisto is out of the image and to the right.

Q & A: Understanding the Discovery

1.
Why don't triple eclipses on Jupiter occur more often?

Io, Ganymede, and Callisto whip around Jupiter at different rates.
So it takes time for them to catch up with each other. One reason why
the moons orbit at different rates is that they are not all at the same
distance from Jupiter. Their orbits are nested like Russian dolls. Io
is the closest, followed by Europa, Ganymede, and Callisto. These are
the largest of Jupiter's moons. The moons closest to Jupiter travel
faster than those farther away. For example, during the time it takes
Io to complete four orbits, Europa completes two orbits, and Ganymede
one orbit. The ratio between the orbital periods of these three moons
means that they can never be on the same side of Jupiter. Only Io,
Ganymede, and Callisto can line up on the same side of Jupiter.

2.
Why do the shadows of Io, Ganymede, and Callisto fall on Jupiter?

The moons orbit in nearly the same plane around Jupiter because they
were formed from the same disk of debris that created the solar system.
These moons were the first spotted by Galileo in the early 1600s. It was
the first observational evidence that a body could be gravitationally
bound to a system other than that of the Earth.